UAM & the ICAO Balanced Approach – What are the challenges?

Introduction

The first scheduled commercial air service was flown in 1914 by the St. Petersburg – Tampa Airboat Line. By the mid-1950s communities in the vicinity of US airports began to complain about the level of noise. The International Civil Aviation Organisation’s (ICAO) first aircraft noise standard (ICAO Annex 16, Volume 1, Chapter 2) was introduced in 1972. Will the aviation industry, regulators, national governments, and international governing bodies be able to define and agree a new set of noise standards before the emergence and wide scale commercial operation of Urban Air Mobility (UAM) aircraft? What can be done to minimise the community noise impact of UAM ports and drone delivery hubs?

UAM aircraft will provide a new era of air travel, probably the greatest change since the Wright brothers first “heavier than air” powered flights in 1903. The revolution in our skies will see UAM aircraft, capable of vertical take-off, providing air-taxi services for passengers and cargo across and between cities. There is still safety, security, legislative and regulatory hurdles to overcome but equally there are social acceptability challenges. Noise may prove to be one of the more difficult challenges to mitigate.

UAM commercial services may start as soon as 2024, potentially as piloted operations but the expectation is that they will become unmanned autonomous operations in time. In 2019 a study identified total forecast investment in UAM infrastructure (vertiports, traffic management technology) and vehicles over a 20-year period of $318bn[1]. The UK Government is supporting the development of this future air revolution and one of its many investments in this domain is helping to deliver an experimental air hub in Coventry which opened on 25^th April 2022[2]. It is anticipated prior to the commencement of commercial UAM operat#reference1ions that small sized package delivery and aerial surveillance drones are heard and seen in our neighbourhoods.

The revolution in our skies is a challenging use-case in urban and residential environments due to public perceptions over privacy, noise, and safety; equally some see the revolution as a means to cut climate damaging emissions through a reduction in delivery vans. A recent study concluded that in general Unmanned Aerial Vehicles (UAVs) produce significantly lower emissions compared to ground delivery per parcel-kilometre; up to 35% lower compared to electric road vehicles[3]. To become as integral to our transportation system as cars, UAM aircraft will have to transition from being technologically possible to commercially viable and socially acceptable. Whilst, drones are already a proven technology they have yet to achieve large scale urban operations and social acceptability. The primary focus of this paper is on the challenges facing drones and UAM from a perspective of environmental noise and what needs to be achieved to ensure appropriate standards are in place to mitigate social annoyance and rejection.

 

Our skies are changing

With Skyports, a UK company, using drones to fly COVID medical supplies between hospitals in Argyll and Bute[4] operating beyond visual line of sight (BVLOS) with CAA approval a UK first was achieved, evidence that the UKs airborne delivery sector is emerging. Further evidence comes from the CAA’s approval in April 2021 for a trial of regular BVLOS drone operations by Sees.ai at three locations within the UK[5]. UPS Flight Forward, announced that it is to buy up to 150 Beta Technologies Alia-250 electric vertical take-off and landing (eVTOL) cargo aircraft, with the first 10 scheduled to arrive in 2024[6]. Both American Airlines and Virgin Atlantic have placed orders for up to 250 and 150 passenger eVTOLs with Vertical Aerospace respectively, whilst Avolon, an international aircraft leasing company, also agreed an order for 500. Virgin Atlantic have identified up to 37 towns and cities within 100miles of Heathrow Airport which would be served by the eVOTL, transferring approximately 7.7m passengers per annum, saving emissions and time compared to driving.

Joby Aviation and Uber are working together to develop a seamless experience through the integration of their respective air and ground transportation offerings. In February 2022 Joby and Japan’s largest airline, All Nippon Airways (ANA), joined forces to establish an aerial rideshare service in Japan; potentially reducing journey times between Kansai International Airport and Osaka Station to less than 15 minutes, compared to over an hour by car. These examples provide evidence of the belief, progress, and investment that is driving this airborne revolution.

The UAM vision is that numerous missions will be flown in and around our cities with individual vertiports serving up to a thousand aircraft per day[7]. The scale of drone operations could dwarf UAM operations with projections of the value of new drones being delivered per annum rising from $520m USD in 2019 to $9.51bn in 2027[8]. One of the many challenges in achieving this vision is that UAM missions will be flown at relatively low altitudes, less than 3,000ft, at speeds of 230mph or less; whilst drone operations will be at even lower altitudes potentially being 200 to 400ft. Both UAM and drone flights will be above populated areas that may not be accustomed to, or expecting airborne noise.

                                                                                                                       

FIGURE 2: EASA RESULTS COMPARISON BETWEEN FAMILIAR SOUNDS AND UAM
VEHICLE SOUNDS AT THE SAME MAXIMUM NOISE LEVEL

The noise challenge

A study by the National Aeronautics and Space Administration (NASA)[9] has shown that drone noise does not resemble qualitatively that of conventional aircraft which introduces an unknown repercussion in terms of the relationship with community annoyance; whether UAMs will have the same or similar qualitative differences in sound is yet to be heard. The NASA noted an interesting effect that increasing the altitude of drone operations between 20 and 100m had little effect on the annoyance relationship despite the overall loudness reducing by 8dB (a 10dB change is perceived by the human ear as halving or doubling of volume); demonstrating that it is more than the pure decibel level that needs to be considered and mitigated. A study by the European Union Aviation Safety Agency, on the societal acceptance of Urban Air Mobility in Europe[10], concluded drone and UAM noise was perceived as more annoying than familiar sounds at the same maximum noise levels. see Figure 2.

 

It is obvious, that without innovative design and a proactive approach by the industry, the new noise exposure and annoyance from UAM and drone vehicles will limit their acceptance and proliferation. A recent study undertaken in has identified that most people in Germany are already against the use of urban airspace for transportation purposes[11]. The EASA study on social acceptability of UAM identified that the environment and noise were the greatest concerns in relation to the operation of UAM but were only ranked third and fourth in concerns for drone operations. A similar study conducted by Airbus found that the top three concerns were safety, type of noise generated from the aircraft, and volume of sound from the aircraft.[12]

 

 

 

Noise certification

This synopsis of aviation history is a powerful demonstration that there is a lot more to achieving permission to operate a new breed of aircraft than just how loud it is. In 1956 Pan American Airways (Pan Am) sought permission to fly Boeing’s first jet airliner the 707 (B707) from New York JFK Airport to Europe. Boeing assured the Port Authority of New York, the owner of the airport, that the Boeing 707 was no louder than an equivalent sized propellor-driven airliner of the era, the Super Constellation. Studies confirmed that whilst both aircraft only produced 112dB at maximum the jet was perceived by people as being far louder. The Super Constellation had its loudest noise in the low-frequency range, 50-200 Hz, while the jet’s loudest noise occurred in the range of 500-2,000 Hz. The human hearing mechanism is many times more sensitive to high- than to low-frequency noise. Further research showed that the jet aircraft would have to be 15dB quieter to be perceived as causing the same noise disturbance as the Super Constellation or put another way one B707 departure would equate to the disturbance caused by 32 Super Constellations. What followed was a two-year battle until Boeing managed to demonstrate that with engine mufflers, a novel climb profile and a turn away from residential areas that they could achieve a noise level that was perceived by residents to be the same as the Super Constellation. International aircraft noise certification was applicable from 1972, 14 years later. Source: Public Demands and Technological Response: Austin Tobin, Leo Beranek and the Advent of Jet Travel, Jameson W. Doig, Journal of Aeronautical History, Paper No. 2017/03, https://www.aerosociety.com/media/6988/public-demands-and-technological-response-austin-tobin-leo-beranek-and-the-advent-of-jet-travel.pdf

 

 

FIGURE 3: DUCTED-FAN CONCEPT

 

Noise certification standards for aircraft are defined within ICAO Annex 16 Environmental Protection Volume 1 Aircraft Noise which are then enacted through local legislation such as US FAA Part 36[13] or EU Certification Specification-36[14]. There is no minimum weight or a requirement for a human pilot within Annex 16 so existing noise certification could apply to UAM aircraft, however there are no appropriate standards currently defined. As it is currently written a UAM aircraft would fall between or across the Annex 16 Chapters that cover, helicopters not exceeding 3,175Kg MTOW, tilt-rotors, and propeller driven aircraft not exceeding 8,618Kg; even then a UAM similar to that proposed by Lilium with 36 ducted fans would be an outlier.

ICAO’s Committee on Aviation Environmental Protection (CAEP) Working Group 1 – Noise (WG1) primary focus is to maintain and ensure ICAO noise certification standards are relevant, effective and to maintain an efficient certification procedure that minimises costs. One aspect of WG1 is to monitor developments around the noise of new entrants such as Remotely Piloted Aircraft Systems / Unmanned Aircraft System, electric/hybrid aircraft, air taxis, eVTOL, etc.

The European Commission through the European Union Aviation Safety Agency has so far only published noise limits for open class drones less than 4Kg in mass[15].

A blog by Cranfield University[16] from September 2019 commented that “there were no specific certification requirements for eVTOLs”. On 16^th April 2021 the Civil Aviation Administration of China (CAAC) announced that it had formed a national eVTOL certification team. The European Union and China have signed a Bilateral Aviation Safety Agreement allowing reciprocal type certification. It is paramount that the primary focus of certification is safety but it will be interesting to see how noise certification develops.

Aircraft noise certification requires aircraft to be operated in configurations that produce maximum noise, these configurations are relatively easy to identify for conventional aircraft as they relate primarily to thrust, weight and drag (through deployment of flaps, air-brakes, and landing gear). As discussed later in this paper, drones and UAM will have a greater number of flight control configurations that can deliver the same flight operation; hence identifying configurations that produce the maximum noise in different flight stages may be impracticable.

It has been expressed that “UAMs need to be 15dB quieter” than helicopters[17]; pure speculation by the author but does the 15dB reduction stem purely from the reduction required by the Boeing 707 back in 1958? ICAO Annex 16 currently requires helicopters not exceeding 3,175Kg to achieve a Sound Exposure Level (SEL) between 82 and 85.5 dBA depending on maximum mass.

The FAA has issued a notice of proposed rulemaking, “rule of particular applicability for noise certification requirements, for the Matternet M2 quadcopter”, because they state “there are currently no generally applicable noise certification standards for this aircraft”[18]. The noise tests specified have been adapted from those for a light helicopter and require noise readings during level flight at 76m (250ft), reduced from the 150m (492ft) for small helicopters, at maximum speed when both empty and at maximum take-off weight. The two different speeds reflect the current uncertainty as to which flight condition will generate the greatest noise. The FAA have calibrated the required SEL based upon both the lower test altitude and the lighter weight on the M2 drone, resulting in a limit of 78dB. In addition to the changes in flight profiles the FAA have also included updated technical specifications for the equipment and environmental surroundings utilised for the testing.

Noise limits are likely to develop as our understanding of UAMs develop but the question is should the ability and cost of technology drive the limit, or should the human experience drive what the technology has to achieve? In the past we understood far less about the human perception and impact of aviation noise, so previous noise limits in Annex 16 were driven by technology and cost. With our greater understanding should we change the way limits are set to ensure that new technology, such as UAMs, are sustainable in terms of their direct and indirect noise impacts. If it is to be the case that perception and impact orientated noise standards are to be set, then UAM designers and engineers must have the tools necessary to predict and assess noise throughout the UAM development stages.

NASA are undertaking extensive research for the development of; noise prediction tools and noise reduction technologies; ground and flight test methods; development and understanding of the human response and metrics; and regulations and policy.

 

ICAO Balanced Approach

Despite the noise certification of aircraft, noise has continued to plague aviation. ICAO therefore formulated its Balanced Approach[19] to Noise Management as industry guidance for the reduction and mitigation of noise. The Balanced Approach has four defined pillars;

i. reduction of noise at source,
ii. noise abatement operational procedures,
iii. land-use planning and management, and
iv. operating restrictions.

A fifth pillar of community engagement is not specified but is widely acknowledged as the most critical aspect to managing the impact of aircraft noise. To enable the future aviation revolution, drone and UAM developers need to know what noise levels they need to achieve and then we need to achieve the following pillars of the Balanced Approach if the industry is to avoid the imposition of “operating restrictions“ (pillar 4);

• • • minimise noise at source – innovative engineering, design, and materials;
    • noise abatement operational procedures – through identification of optimal flight practices using advancements in predictive tools;
    • land-use planning and management – by comprehending the impact through new noise and annoyance models to enable a pro-active rather than re-active approach; and,
    • “community engagement” and communication programmes – to build awareness, understanding and trust to gain social acceptance.

 

Minimising noise at source

The primary tool used to drive reductions in noise at source in aviation is the noise certification standards for new or modified aircraft. ICAO has over the years reduced the certification noise thresholds driving manufacturers to seek technology improvements to make aircraft quieter. Today’s modern fleet of commercial jet airlines are 90% quieter than the first generation such as the Boeing 707.

Historically the variation in design between different manufacturers and categories of fixed and rotary wing aircraft has been relatively limited resulting in very similar noise profiles within commercial jet aircraft, helicopters, or light aircraft. From the drones already on the market and the UAM concepts and prototypes presented there are likely to be many variations in noise profiles. For vertical take-off three options are either multiple open rotors, enclosed rotors, or ducted fans. Rotors on drones operate at variable speeds unlike conventional helicopters, this may or may not be the case on UAM aircraft depending on the mass of the blades. Some UAS designs are presenting different propulsion systems for different functions, for example the UPS Flight Forward Alia-250 has four rotors for lift and one rear mounted propellor for propulsion. A rear mounted push propellor tends to be noisier than a forward mounted pull / tractor propellor, due to the interaction of the airflow from the airframe with the propellor.

 

FIGURE 4 : POTENTIAL NOISE SOURCES ON A ROTOR BASED UAM AIRCRAFT

The characteristics of rotors and small electrical ducted fans will change the acoustic frequency content and temporal[20] features of drones and UAM aircraft relative to conventional rotary and fixed wing aircraft. Design factors that can affect noise at source on UAM vehicles include, number of motors, motor speed, rotor spacing, blade spacing, blade length, rotor phasing, active control systems, exterior liners, isolation of motor vibrations and reduction of acoustic radiation efficiency of vibrating surfaces. Additionally, as with rear mounted push propellors, the interaction between the disturbed airflow around the airframe and the rotors will impact the characteristics and loudness of the noise.

It is not just the loudness of the noise that is important, as demonstrated by the synopsis of aviation history and the Boeing 707 above, the tone and temporal noise are also important. Psychoacoustics is concerned with the perception and audiology of sound, how humans perceive various sounds. If buying a sports car, you are unlikely to want to hear the whine of an alternator but hearing and potentially feeling a deep throaty roar from the engine and exhaust may be an important element of the experience; hence why the automotive industry has invested extensively in psychoacoustics. The application of psychoacoustics within UAM and drone design could ensure that the sound produced is less annoying, for example, optimising the position of rotors[21].

Choices made by designers is fundamental to the ability to minimise noise at source. Choices at component level influence the overall noise at the aircraft system level. At the system design stage quick assessments that indicate trend rather than absolute values are desired and empirical models are currently too expensive, so surrogate models need to be used, however surrogate models require measured data or other complex computational models to feed them[22]. For novel aircraft systems and their components little or no empirical acoustic data exists to enable current component and system level noise predictive tools[23] to be validated; making design decisions that much harder.

Given the number of drones already on the market and the speed of developments with new drones and UAM aircraft an ability to predict component and system noise is probably overdue. There may be scope to use existing models for assessing some components or aircraft system noise attributes, however it is critical that research and trials are conducted to validate or inform updates to existing models or drive the rapid development of new models. The challenges here are two-fold, the first being the need for full-scale proto-types to facilitate robust research whilst the other is the time it takes to deliver the research to a level of fidelity that can be efficiently integrated into the design process.

At the start of September 2021 Joby Aviation and NASA commenced acoustic data capture utilising Joby’s full-scale eVTOL prototype and NASA’s mobile acoustic facility, a first within NASA’s Advance Air Mobility National Campaign. Once complete Joby and NASA will have the first noise hemispheres for an eVTOL UAM that captures the true intensity and character of the sound emitted, enabling comparison with existing drones, helicopters and aircraft. This acoustic data capture needs to be repeated for the multitude of UAMs in development if a comprehensive databank of acoustic hemispheres is to be available to feed noise models and for analysis.

Joby set out to minimise noise at source to create an aircraft with an extremely low noise profile and the current prototype is the culmination of over a decade’s research and development. Failure to minimise noise at source will increase the burden on the remaining pillars of a balanced approach threatening the success of the aviation revolution.

 

Noise abatement

Achieving stable flight requires an aircraft’s aerodynamic configuration to be “trimmed”. With drones and UAM aircraft incorporating multiple and distributed propulsion units, there by multiplying the number of flight control parameters that can be adjusted such as rotor speed or thrust vectoring, the same flight condition may be achieved through different aircraft configurations. The choice of aircraft configuration can have a major influence on the noise generated. The multiple options available within drone and UAS flight configurations may make it difficult to identify low-noise operating procedures for an individual drone or UAM aircraft and extremely difficult to establish generic procedures and/or advice for low-noise operations as has been achieved with Noise Abatement Departure Procedures and Low Power Low Drag approaches for conventional fixed wing aircraft operations.

As with noise reduction at source the availability of suitable noise prediction models is key to achieving the second pillar of the balanced approach, without waiting until these vehicles are in commercial use. To identify noise abatement procedures the predictive models will require capability to predict noise for the whole aircraft system across combinations of operating procedures and flight configurations. Such models are not currently available thereby risking social rejection before the benefits can be truly understood and appreciated.

 

Land-use planning and management

In the previous two sections we have looked at reducing the noise of the aircraft either through design or operating practices, i.e. the generation of the noise. Now we turn to look at how to mitigate the effects through controlling the population impacted.

Land-use planning and management requires us to understand the propagation of the noise to the ground and the way it is perceived by communities. To assess the community noise impact, predictive models are required but they do not require the same fidelity as the models necessary for assessment of individual components or a single aircraft system. Community impact models do need the ability to model many aircraft types and a large number of flight operations.

 

The models

Community noise modelling tools such as EUROCONTROL’s IMPACT, UK’s Aircraft Noise Contour model (ANCON) or the US FAA’s Aviation Environmental Design Tool (AEDT), used for current fixed wing and Europe’s Noise of Rotorcraft Assessed by a Hemisphere-approach (NORAH) model used for rotary wing aircraft, do not encompass the performance data for drones and UAM aircraft. In the near-term it may be possible to make noise estimates by developing lateral directivity assessments, but these are likely to be specific to a single UAM and fixed flight condition, rather than being a dynamic trajectory and thrust based approach. As further UAM aircraft come to fruition and accurate acoustic measurements are gathered for drones and UAM aircraft alike then the data necessary to identify relationships between aircraft configuration, phase of flight and noise may be established and embedded within existing or new community noise impact tools.

Research undertaken by the NASA Aeroacoustics Branch and presented at Inter Noise 2021 has demonstrated that within the existing capabilities of AEDT it is possible to generate community noise impacts for representative UAM aircraft, to a limited extent. The research utilised AEDT’s fixed-point flight profiles, only available within AEDT currently for fixed wing aircraft types, and assigned user supplied noise data; thereby avoiding the need for an AEDT aircraft performance model and aircraft noise data. The results were limited by the lack of a source directivity definition and hover capability for fixed-wing aircraft types. Additionally, the empirical generation of the extensive user-supplied noise data is not something that is practically achievable outside of a research community currently.[24]

 

Thresholds and Metrics

The importance with community noise impact assessment is utilising metrics that have been shown to relate well to surveyed annoyance levels, which is difficult given the small-scale use of drones and no large scale UAM trials. Hence in the interim we will have to consider applying metrics and threshold values used elsewhere within environmental noise management, although studies by NASA and EASA indicate less tolerance therefore greater annoyance from those exposed to UAM and drone noise.

In the UK daytime noise annoyance is assumed to start at 51 dB LAeq, 16hour and night time noise at 45 dB LAeq, 8hour. These are called the Lowest Observed Adverse Effect Levels (LOAELs). In the UK the onset of significant annoyance from aviation noise is considered to begin at 57 LAeq 16hour, although the “Survey of noise attitudes 2014: Aircraft”[25] presented evidence that it had reduced to 54 LAeq 16hour. The UK Government expects airport operators to assist households with moving if their property is exposed to noise greater than 69 LAeq 16hour and to provide assistance with sound insulation for noise sensitive buildings, e.g. schools & hospitals, exposed to noise greater than 63 LAeq 16hour. Many airports have voluntarily helped financially with sound insulation to residential properties at levels below 69 LAeq 16hour.

In relation to rail, new lines and additional tracks that cause noise to exceed 68 dB LAeq 18hour during the day or 63 dB LAeq 6hour at night, entitle the property owner to support with sound insulation. There are also discretionary powers to assist with sound insulation if rail enhancements cause the noise level to increase by 1 dB(A) above the prevailing level prior to the works[26].

UK property that is affected or expected to be affected by road noise, triggered by a new or modified highway, greater than 68 dB(A) L10 18hour[27] where the increase caused is at least 1dB(A), is entitled to assistance with insultation works[28]. The L10 18hour metric could be seen as more appropriate than the LAeq metrics used for aviation and rail as the L10 considers the 6-minutes with the greatest average noise level from an hour rather than the average noise across the whole hour; thereby being more focus to peaks within average noise.

Internationally the Effective Perceived Noise Level (EPNL) metric is stipulated in ICAO Annex 16 for the noise certification of fixed wing aircraft and stems from the studies by the Port Authority of New York in the late 1950s when evaluating the noise of the first jet airliners. The EPNL accounts for the perceived noise level, which itself is derived from a combination of amplitude and frequency, as well as containing a correction to account for the duration of exposure and a tonal penalty; the EPNL may more closely represent the psychoacoustic experience of communities.

Community members do not understand or relate to metrics based on equivalent sound principles as individuals hear and respond to specific noise events and not average noise levels. The use of noise metrics related to the number of events are easier for community members to understand and relate to as they are more closely aligned with how we notice noise events, e.g. Number of aircraft noise events above 65dBA (NA65). Noticeability can be affected by the tonal and temporal aspects rather than just the loudness, it is also heavily influenced by what someone is doing, the environment they are in and the ambient noise levels. The relationship between noticeability such that the noise intrudes and triggers a physiological or emotional response is not straight-forward, hence why the majority of the research uses equivalent noise metrics rather than event based metrics to assess the noise dose.

 

Human Response

Once we can model drone and UAM aircraft noise to produce community scale noise contours it will still be necessary to understand the human responses.

Since drones and UAM aircraft are expected to operate in much closer proximity to people than conventional aircraft the noise may be more noticeable and therefore regarded as more intrusive and subsequently more annoying. The urban environment, in which drones and UAMs will tend to operate, may help to disguise the noise although the reflection of noise off buildings and the possible channelling of it down streets with high rise buildings may increase the number of people noticing the noise whilst making it harder to locate the source; how these factors will affect perceptions of UAS noise is not clear.

Our understanding of the human response to noise for conventional aviation is expanding year on year. Our ability to predict community response is based on the development of dose-response functions that quantify the relationships between noise exposure and the human reaction; from the development of the Schultz Curve in the 1970’s, that related transport noise to the percentage of the population highly annoyed, to the more recent exposure-response functions (ERF) for relationships between aircraft noise and children’s learning or cardiovascular disease. In developing these dose response functions, the aircraft noise metrics are almost always based on daily or annual equivalent sound exposure utilising an A-weighted scale. Integrated sound exposure is based on the principle of equivalent noise energy i.e. the number, decibel level and duration of events are interchangeable if the total sound energy remains unchanged, see Figure 5. The A-weighted scale is a standard frequency-weighting that correlates well with the perceived loudness in many instances. A-weighted metrics do not account for some of the characteristics of the noise produced by drones, such as tonal components, rapid fluctuations in the sound levels and both low and high frequency noise; which are expected to be present but to a lesser extent within the noise from UAM aircraft. The forecasts for large numbers of UAM aircraft operations (in some projections, up to two per minute) at busy vertiports exceeds the movement numbers at all but a few of the busiest airports during peak hours, but significantly less than vehicle noise events on major roads where the use of A-weighted and equivalent sound metrics are widely accepted and used within ERF.

There is a very limited set of sound data from UAM operations currently, but it is anticipated that the noise characteristics will be substantially different to current fixed and rotary wing aircraft as has been the experience from smaller drones, meaning that the use of existing models is unlikely to be appropriate. The research that is underway is developing empirical noise models for multi-rotor aircraft and is beginning to validate their results against data from early UAM prototypes.  Ultimately, trials will need to be conducted with UAM aircraft in real-life environments to gather the data to extend or replace existing noise models. The trials will provide initial understanding as to whether existing noise metrics will need to be replaced and if they do, new dose-response relationships established between drone and UAM noise, and annoyance and health impacts.

 

Community engagement

Community engagement will not do anything to reduce the loudness of a noise, yet it can have positive effects on reducing the amount of annoyance generated[29], hence the importance of effective engagement and communication cannot be overstated. When communities experience an unexpected change in their sound environment there is often an increase in resentment and anger towards the noise source; this has been experienced time and again with poorly communicated changes by airports and air navigation service providers. Communities that are newly affected by airborne noise have shown themselves to be far more sensitive, energised and determined in their response to noise polluters and much harder to pacify. So, engaging with communities on what wide scale drone and UAM operations will mean for them is fundamental.

Airport noise management programmes are focused on the communities in the vicinity of the airport and those under heavily used lower level (typically less than 8,000ft) flight paths. With drones and UAM aircraft operating across wide urban areas, with potentially competing vertiport and UAM operators, coordinating the community outreach programme will be much harder and is likely to need to reach many more households.

In a similar vein it is fairly easy for an individual to log a noise complaint in relation to existing airspace operations, as it is easy to associate the flight with a nearby airport. With remote drone operations, multiple vertiports spread across an urban environment, and large numbers of UAS operations then identifying the noise offender and linking it to the relevant organisation will be challenging for individuals. Reporting drones and UAMs will reach beyond noise and will almost certainly include invasion of privacy reports, the public concern about an invasion of privacy by “snooping” drones, as demonstrated by newspapers headlines, reaches back over five plus years[30]. It is perfectly reasonable for communities to expect it to be easy to report noise or privacy events as part of a robust governance process for holding operators to account. Back in 2005 residents in London spoke out about the challenges in reporting helicopter noise events as it was, and still is 17 years later, unclear as to who is responsible for effectively handling such complaints[31][32]. Communities around airports have regularly made the accusation that they get passed between the multiple stakeholders involved in aircraft noise as a deliberate attempt to undermine their efforts to seek resolutions. Airports and airlines have experienced that losing the goodwill and trust of communities can have far reaching impacts on their businesses with many airports now understanding the importance of maintaining a “social license to operate” to avoid the imposition of operating restrictions and other punitive measures.

It is important that the UAS industry, regulators and authorities work together collaboratively to identify responsibilities and boundaries. Without clear lines of responsibility community engagement will be fragmented, and communities will not know who to contact for registering complaints and gathering information.

 

Conclusions

Airborne transportation, of packages and people, is on the cusp of a major revolution with the emergence of small-scale drones used for delivery and surveillance, and large scale eVTOLs for passenger transport across cities and between regions. The reality of BVLOS trials, orders for large numbers of UAMs and the forecast investment is evidence that there is significant confidence that this revolution is starting. One of the more difficult challenges in gaining certification and social acceptance of the new aerial vehicles is expected to be noise.

This paper has identified that there are a number of gaps that are unlikely to be filled before the first UAM and drone ports open and the new UAM aircraft are operating in urban environments.

1. 1. There are no internationally recognised noise certification standards for drones and UAMs, other than for small drones weighing less than 4Kg.
   2. There is a lack of knowledge as to how many and what classes of UAM aircraft and drones will be required before consistent and repeatable flight configuration parameters for noise certification can be defined.
   3. UAM and drone noise prediction models at the component, system or community impact level are not at a level of development and sophistication to facilitate use outside of research environments.
   4. There currently is insufficient evidence to understand the physiological and cognitive impacts of drone and UAM noise on society or to develop dose-response relationships, thereby increasing the likelihood that standards will be defined by the limitations of technology and affordability rather than what is safe from a health impact perspective and what is socially acceptable.

In the absence of noise certification standards, noise modelling tools, and the social research on the impact of UAM and drone noise, what can the stakeholders in this air revolution do to maximise the social acceptance from an acoustic perspective? I have provided, below, some thoughts on what actions stakeholders could be doing:

 

Government

The Government are responsible for formulating and updating policy. Three aspects that warrant consideration to assist with the introduction of UAM and drone deliveries are:

1. 1. Direction as to whether UAM and drone noise will be treated as per aircraft noise and therefore not classified as a statutory nuisance.
   2. Guidance on whether the approval of UAM ports and drone delivery hubs from a land-use planning, and by extension from a community noise perspective, will be the accountability of the local planning authority or the CAA.
   3. Clarity on which party, aircraft operators, port operators, local authority or CAA, will be responsible for receiving, handling, investigating and responding to UAM and drone noise and privacy complaints, particularly once multiple aircraft and port operators are active in a single urban environment.

 

Regulator

With or without noise certification criteria regulators are going to have to determine how they will handle applications for UAM and drone ports as well as airspace change proposals for UAM and drone operations. It is therefore recommended that regulators:

1. 1. Develop policy and guidance on the approval process and requirements for UAM ports and drone delivery hubs in an urban environment. In developing the policy and guidance the regulator needs to consider if existing environmental noise guidance and practices should or could be applied. In the UK there is not a specified threshold for what is considered an environmental noise nuisance, instead local authorities consider each instance on its own merits. However, in the UK we understand that the lowest observable adverse effect from aircraft noise commences at 51dB LAeq by day and 45 dB LAeq by night, could these levels be used as guidance?
   2. Establish a data bank of UAM and drone noise recordings. To maximise consistency and comparability in noise recordings it would be beneficial if the CAA were to publish guidance on appropriate methodologies to capture noise recordings of UAM and drones, and to outline flight procedures and / or flight configuration parameters under which recordings should be taken.
   3. Determine the criteria at which drone and UAM operations trigger the need for an Airspace Change Proposal that in turn requires noise assessment. For example, it would not be anticipated that a single drone undertaking urban photography would need an ACP, but for more permanent or heavily trafficked areas such as drone delivery hubs or UAM ports it would appear to be warranted, however where is the cut off between the two?
   4. The existing CAA guidance on airspace change requires an assessment of the population to be impacted by noise and a monetisation of the impacts to be conducted, for flight paths at lower altitudes. If UAM and drone noise models are not available alternative approaches to the evaluation of airspace changes will be required and consideration as to whether existing or updated WebTAG guidance is appropriate.

 

Manufacturers / Operators

Manufacturers of UAMs and drones, as well as operators of the aircraft, and the UAM and drone port operators, can ease their future approvals by acting now.

1. UAM and drone manufacturers should ensure they develop robust evidence of the noise profile of their vehicle spanning the operational capability of the aircraft in different operating environments and meteorological conditions. Vehicles intended for the urban environment should be designed with minimum noise as a primary design criterion in line with the ICAO balanced approach.
2. Manufacturers and operators of the aircraft, and the ports could work on the development of low noise arrival and departure operating protocols. This would demonstrate a proactive noise mitigation culture to regulators, authorities, and communities.
3. UAM and drone port operators could be investigating, developing, and trialling noise monitoring systems, to ensure that noise monitoring in the vicinity of the ports is possible and appropriately calibrated from the start. This will be a vital data source to enable data driven information sharing and robust engagement with local communities and authorities.

When the first commercial UAM services enter service, it will be almost 70 years since Pan Am commenced jet airliner operations from New York to Europe, with the Boeing 707. Technically the Boeing 707 was no louder than the loudest non-jet airliners of the day and communities had accepted them; but New York communities certainly did not accept the Boeing 707. If Governments and Regulators do not rapidly define noise standards, which take full account of; any differences in the quality of the sounds between conventional aircraft and the UAMs of tomorrow; and the perception of the new noise by society; then aviation noise history is about to repeat itself, endangering the social acceptance and wide adoption of the next air transport revolution.

Author: Nick Boud, Principal Consultant, Think Research

 

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Sources

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[20] Temporal features of sound affect auditory perception, including loudness, pitch and timbre perception and spatial hearing
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[23] For conventional aircraft models such as NASA’s Aircraft Noise Prediction Program 2 (ANOPPs), or a combination of three tools developed by DLR’s (Deutsches Zentrum für Luft- und Raumfahrt / German Aerospace Centre)

• TAU-Code: a system for complex flow simulations on unstructured hybrid grids
• Perturbation Investigation of Aerodynamic Noise (PIANO) software for computing the noise emission at component level
• Parametric Aircraft Noise Analysis Module (PANAM) tool computes the noise emission of entire aircraft for different flight configurations.

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